737 NG auto speed problem

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737 NG auto speed problem

Postby Technar » 04 Feb 2019, 18:52

Hi friends,
I have problem with auto speed brake sys of NG. When deployng on landing the auto speed brake actuator drives the lever to up position and then it continues to run. The actuator is changed with no result. Apart from this sometimes the actuator is not able to push the lever from FLIGHT detent to UP detent. The force necessary to pull the lever manually through the full range is within the AMM limits.
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Joined: 02 Nov 2013, 18:19

Re: 737 NG auto speed problem

Postby sjap » 04 Feb 2019, 19:52

And the test with the module in the EE comp. Does it pass the test? Try with disconnected actuator, just loose from the mechanism. Does it work then?
Still looks like a heavy control cable to mixer problem?
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Re: 737 NG auto speed problem

Postby maro12 » 05 Feb 2019, 12:03

Hi Technar,

If actuator is still operating when handle is raised, it means internal switches supplying RAISE coil in actuator did not open. This might be caused generally by internal switches fail closed, or actuator is not raised enough to open the switches. You replaced the actuator, so with high probability eliminated internal switches fail close case.

The actuator would not rise enough to open internal limit switches because: reaching external mechanical limit before internal limit switches open, or would not have power to overcome mechanical load to reach extended limit.

Do the Speed Brake Throttle Switch Test TASK 27-62-00/501 latest revision, and see how system behaves.

First thing I would do, is to check actuator disconnected from mechanism as per Sjap instructions. This is new actuator so it should work properly - it should extend fully and motor should stop.

Second thing I would check actuator adjustment as per Installation procedure, Ref.AMM 27-62-31/401 latest revision.

I would also have a closer look on the control cable to mixer forces, as per Sjap.

Furthermore I would check power in to the actuator (between pin 1 and 7) with multimeter and proper test lamp to check how it works with some load. If actuator can not overcome normal operating forces, might be due to power quality to actuator. This power comes from Auto Speed Brake Module through R708 Spdbrk Actuator Enable Relay and R01010 Diode. I would check R708 resistance when energised (check relay on the bench), diode and measure wiring. Auto Speed Brake Module might also give lo power out, if You have possibility I would arrange to swap it for testing.

I would also ask:

-When deploying in landing, does the lever go to full mechanical stop (You can not move it any further back by hand, even small amount of movement - apart of cases when not able to push from FLIGHT detent to UP detent)?

-After deployed during landing, when does actuator stop to run (what action do You do to stop it)?

I am also thinking, if actuator is still running against mechanical resistance many times, how long it will stand without failing.

Let us know the findings.

Best Regards,
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Re: 737 NG auto speed problem

Postby motortek » 08 Feb 2019, 03:53

u must try another actuator ..
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Joined: 16 Dec 2011, 02:05

Re: 737 NG auto speed problem

Postby Technar » 11 Feb 2019, 22:54

I agree with motortek. I suspect faulty spare actuator. My idea is that the actuator is not able to overcome the friction of the system. When this occurs during landing the actuator stops before reaching UP detent. In this moment the pilot grabs the lever und pulls it to UP detent. This action dissynchronizes the positions of the lever and the actuator - the lever reaches the UP stop while the actuator has not reached its up limit switch. This happens because they are operated simultaniously. In normal practice this never happens. What I wanted to try is during the test when the actuator is driving the lever towards UP detent me to move the lever slowly towards DOWN detent. In this way the no-back mechanism will allow the actuator to reach its UP position. Then I would set the lever to UP pos. thus synchronizing both the actuator and the lever. Of course this wouldn't solve the problem with the unability of the actuator to overcome the lever and the system. I didn't have chance to try it becausee the aircraft moved to another base.
If i get further information I will share it .
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Joined: 02 Nov 2013, 18:19

Re: 737 NG auto speed problem

Postby Technar » 18 Feb 2019, 17:44

As expected after second actuator replacement everything is OK.
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